This invention relates to aldol condensation processes and in particular to the condensation of ketones.
The coupling reaction of relatively small molecules to form relatively large molecules is a commercially attractive route to form a range of products having specific structures and properties. As an example, Methyl isobutyl ketone (2-methyl-4-pentanone) is the largest volume aldol reaction product of acetone. Methyl isobutyl ketone (MIBK) is an excellent solvent for cellulose and resin based coating systems and also for vinyl, epoxy and acrylic resins. It is traditionally manufactured via a three-step reaction scheme as described in Industrial Organic Chemistry, 3rd Edition (eds. K Weissermel and H J Arpe), Wiley (1997), p 280-281, the stages comprising (i) the base catalysed aldol condensation of acetone in the liquid phase to diacetone alcohol (DAA), (ii) the acid catalysed dehydration of DAA to mesityl oxide (MO) and (iii) the hydrogenation of MO to MIBK and further to methyl isobutyl carbinol.
These processes are complicated and the operating costs are high. The condensation equilibrium in step (i) does not favour aldol formation. In step (ii) acetone can be formed by the reaction of mesityl oxide with water and in step (iii) the less useful methyl isobutyl carbinol has to be separated by distillation. There is also a corrosive problem due to the use of liquid acids and bases.
Recently, a one-step process from acetone to MIBK has become commercially feasible and several catalytic systems have been described for this process. They mainly consist of palladium supported, for example, on KOHxe2x80x94Al2O3, MgOxe2x80x94SiO2 or cation exchange resins (Kirk-Othmer Encyclopaedia of Chemical Technology, Vol. 13, Wiley, New York, 1979, p.907), CaOxe2x80x94MgOxe2x80x94SrOxe2x80x94Al2O3 as described in JP-A-62258335, Nb2O5 as described in JP-A-63096147; ZrO(OH)2-carbon as described in JP-A-63068538 and Ce, Hf and/or Ta oxides or hydroxidescarbon as described in JP-A-63096146. Very high selectivities to MIBK ( greater than 90%) are described in the 80-160xc2x0 C. range and acetone conversions near 40%. The high operating pressures required, typically 10-100 atm are a disadvantage of the single-step process.
GB-A-921510 describes a liquid-phase process for the condensation of acetone to make mesityl oxide using a catalyst which is an alkali-treated activated alumina. The process is favoured at low temperatures, between about 80 and 150xc2x0 C.
Tanabe et al (Applied Catalysis 48 (1989) 63-70) describe the effect of various metal cations on the activity of magnesia catalysts in the liquid phase aldol condensation of acetone.
More recently, catalysts which operate efficiently in the gas phase at atmospheric pressure have been developed for the one-step process. These have included Pd/SAPO-34, described in U.S. Pat. No. 4,704,478, Pd/KH-ZSM-5 (in U.S. Pat. No. 5,059,724); Ni/MgO (L M Gandia et al, Appl. Catal. A: General, 101 (1993) L1-L6), Ni/ALPON (L M Gandia et al, Appl. Catal. A: General 114 (1994) L1-L7; Na/Pd/MgO (K Lin et al, Appl. Catal. A: General 147 (1996) L259-L265); Ni/Al2O3 (S Narayanan et al, Appl. Catal. A: General 145 (1996) 231-236) and Pd or Ni supported on Mg/Al hydrotalcites (Y Z Chen et al, Appl. Catal. A: General 169 (1998) 207-0214).
U.S. Pat. Nos. 4,086,188 and 4,165,339 describe the gas phase condensation of aldehydes and ketones, especially acetone in the presence of catalysts comprising a complex magnesium-aluminium oxide-hydroxide mixture which is doped with lithium ions. The reactions produce isophorone and mesityl oxide and achieve isophorone: mesityl oxide ratios greater than 1.
U.S. Pat. No. 4,599,453 describes the single stage production of higher aliphatic ketones by reacting a starting ketone with carbon monoxide in the presence of a catalyst comprising copper supported on a metal oxide.
U.S. Pat. No. 5,055,620 describes a polymorphic magnesium-oxide-pseudoboehmite composition for the aldol condensation of acetone to isophorone.
In WO-A-00/31011, the aldol condensation of aldehydes in the gas phase was described using catalysts comprising an alkali metal on an inert support.
We have now found that the condensation of ketones can be effected in the gaseous phase using a solid base catalyst thereby avoiding the need for aqueous caustic solutions with their consequent handling and effluent disposal problems.
It is an object of the present invention to provide a method of forming a carbonyl compound by the aldol condensation of at least one organic ketone. It is a further object of the invention to provide a method of making a saturated ketone or an alcohol by the hydrogenation of a carbonyl compound formed by the aldol condensation of at least one organic ketone. It is a further object of the invention to provide a catalyst which is capable of catalysing the aldol condensation of at least one ketone to form a higher organic ketone.
According to the invention, we provide a process for the production of a product ketone containing at least six carbon atoms from at least one feedstock ketone by contacting the feedstock ketone in the vapour phase with a particulate catalyst comprising at least one basic alkali metal compound supported on an inert substrate at a temperature above 175xc2x0 C.
According to a second aspect of the invention, we provide a process for the production of an alcohol by the hydrogenation of a ketone containing at least six carbon atoms which has been formed from at least one feedstock ketone by contacting the feedstock ketone in the vapour phase with a particulate catalyst comprising at least one basic alkali metal compound supported on an inert substrate at a temperature above 175xc2x0 C.
According to a third aspect of the invention, we provide a catalyst for catalysing the aldol condensation of a ketone at a temperature above 175xc2x0 C., said catalyst comprising at least one basic alkali metal compound supported on an inert substrate.
Suitable catalysts are basic sodium, potassium, or cesium compounds such as oxides hydroxides or carbonates supported on a material such as carbon, silica, alumina, a clay, silicalite or a zeolite Preferred catalysts are alkali metal compounds supported on silica, especially potassium or sodium supported on silica. The potassium and sodium catalysts appear to have high activity and are the most selective. The catalyst preferably contains 0.1 to 25%, preferably 0.4 to 18%, by weight of the alkali metal.
The support preferably is in the form of particles having maximum and minimum dimensions in the range 0.5 to 10 mm, preferably 1 to 4 mm, and having a BET surface area in the range 50 to 500 m2/g. The catalyst is preferably made by impregnating the support particles with an aqueous solution of an alkali metal compound that is basic or decomposes to a basic compound upon heating, for example an alkali metal hydroxide, acetate, oxalate, nitrate or carbonate, followed by drying and calcination if necessary to effect decomposition to a basic compound.
The reaction is effected at temperatures above 175xc2x0 C., particularly above 200xc2x0 C., and preferably below 450xc2x0 C., particularly in the range 200 to 350xc2x0 C. As the temperature increases the activity increases but the selectivity tends to decrease, often with the production of hydrogenated products.
After a period of operation, the activity of the catalyst tends to decrease through the deposition of carbon as a result of side reactions. The catalyst may be periodically regenerated by burning off the carbon by heating in an oxygen-containing atmosphere, e.g. air or oxygen or air diluted with an inert gas such as nitrogen. The catalyst may be disposed as a fixed bed or a fluidised bed may be employed. In the latter case a portion of the catalyst may be continuously withdrawn and regenerated and returned to the reaction zone.
The main product of the condensation is an unsaturated ketone. Often it is desired to hydrogenate the product to the corresponding saturated ketone or its corresponding saturated alcohol. This may be effected by passing the products, possibly after separation of the starting ketone that has not reacted, together with hydrogen, through a bed of a suitable hydrogenation catalyst, such as copper or a platinum group metal, on a suitable support The temperature at which the hydrogenation is effected will often be below that used for the aldol condensation. The reaction mixture from the aldol condensation may be cooled to the desired hydrogenation temperature by addition of a suitable quench gas, such as cool hydrogen.
As indicated above, the primary product from the condensation is the unsaturated ketone, e.g. mesityl oxide. In some cases, the desired product is not the corresponding alcohol but is the corresponding saturated ketone, e.g. MIBK. The unsaturated ketone may be hydrogenated to the saturated ketone using a hydrogenation catalyst, such as palladium, that selectively hydrogenates the carbon-carbon double bond compared with the carbonyl group. This means that the catalyst effects hydrogenation of the carbon-carbon double bond but does not effect hydrogenation of the carbonyl group to any significant extent. Suitable catalysts may be easily selected by the skilled person.
In some cases it may be possible to effect the condensation and hydrogenation in a single stage by formulating the base catalyst also to have the appropriate hydrogenation activity and co-feeding hydrogen with the ketone to the reaction zone. Such a base/hydrogenation catalyst may be a mixture of separate particles of base and hydrogenation catalyst, or may be particles of the support impregnated with both a base and a material having hydrogenation activity. However, it has been found that where the condensation and hydrogenation is effected in a single stage, e.g. by the use of a catalyst having both the condensation and hydrogenation activity, the condensation activity of the catalyst may decrease relatively rapidly. Therefore it is preferred to effect such condensation and hydrogenation in separate stages, e.g. by using a bed of the condensation catalyst, followed by a bed of the hydrogenation catalyst. In this case it is preferred that the condensation catalyst is free from components, such as copper, and Group VIII metals, giving hydrogenation activity.
Regioselective reactions of unsymmetrical ketones are of fundamental importance in organic synthesis, the most familiar reactions being xcex1-alkylations, Michael additions and aldol condensations. In the presence of a base, methyl ethyl ketone, the simplest of the higher ketones, is capable of losing a proton from the adjacent methylene group or the terminal methyl group. Tautomerisation of the two possible carbanions formed from the proton abstraction leads to two regioisomeric enolates. The less substituted enolate 1 is formed by 
irreversible kinetic control, whereas those reactions under thermodynamic control usually yield the more substituted product 2. In more favourable cases one regioisomer can greatly predominate in the equilibrium mixture but often the equilibrium constant is not sufficiently high to achieve an acceptable regioselectivity. The enolates formed can then attack a polarised carbonyl group to form a carbon-carbon bond to form two aldol products. We have found that when the feedstock ketone is not symmetrical, e.g. methyl ethyl ketone, so that the aldol condensation is capable of forming either terminal or internal unsaturation, the catalysts described herein are capable of producing a reaction product which is relatively rich in the terminally unsaturated product, i.e. the catalysts are selective to produce mainly one isomer, both cis and trans forms of this isomer being formed.
The invention is illustrated by the following examples.